Jaw crusher is a kind of common mining machine.in a beneficiation plant, the first stage of size reduction of hard and large lumps of run-of-mine (ROM) ore is to crush and reduce their size. Softer ores, like placer deposits of tin, gold, mineral sands etc. do not require such treatment. Large scale crushing operations are generally performed by mechanically operated equipment like jaw crushers, gyratory crusher and roll crushers.

For very large ore pieces that are too big for receiving hoppers of mechanically driven crushers, percussion rock breakers or similar tools are used to break them down to size.

The mechanism of crushing is either by applying impact force, pressure or a combination of both. The jaw crusher is primarily an compression crusher while the others operate primarily by the application of impact

jaw crusher structure

Jaw crushers are designed to impart an impact on a rock particle placed between a fixed and a moving plate (jaw). The faces of the plates are made of hardened steel. Both plates could be flat or the fixed plate flat and the moving plate convex. The surfaces of both plates could be plain or corrugated. The moving plate applies the force of impact on the particles held against the stationary plate. Both plates are bolted on to a heavy block.

Fig.1 shows a sketch of a double toggles and controlled by a pitman. These are commonly used as primary crushers in the mineral processing plant . The size of the feed opening is referred to as the gape. The opening at the discharge end of the jaws is referred to as the set.

Fig1. Double-toggle jaw crushe

Jaw crusher circuit

Primary jaw crushers typically operate in open circuit under dry conditions. Depending on the size reduction required, the primary jaw crushers are followed by secondary and tertiary crushing. The last crusher in the line of operation operates in closed circuit. That is, the crushed product is screened and the oversize returned to the crusher for further size reduction while the undersize is accepted as the product.

Jaw crushers are installed underground in mines as well as on the surface. When used underground, jaw crushers are commonly used in open circuit. This is followed by further size reduction in crushers located on the surface. Open circuit flow sheets showing in fig2.

Fig 2. jaw crusher open circuit

underground, jaw crushers are commonly used in open circuit. This is followed by further size reduction in crushers located on the surface.

When the run of mine product is conveyed directly from the mine to the crusher, the feed to the primary crusher passes under a magnet to remove tramp steel collected during the mining operation. A grizzly screen is placed between the magnet and the receiving hopper of the crusher to scalp (remove) boulders larger than the size of the gape. Some mines deliver product direct to storage bins or stockpiles, which then feed the crushers mechanically by apron feeders, Ross feeders or similar devices to regulate the feed rate to the crusher.

Alternately haulage trucks, front-end loaders, bottom discharge railroad cars or tipping wagons are used. In such cases, the feed rate to the crusher is intermittent which is a situation generally avoided. In such cases of intermittent feed, storage areas are installed and the feed rate regulated by bulldozers, front loaders or bin or stockpile hoppers and feeders. It is necessary that the feed to jaw crushers be carefully designed to balance with the throughput rate of the crusher. When the feed rate is regulated to keep the receiving hopper of the crusher full at all times so that the volume rate of rock entering any point in the crusher is greater than the rate of rock leaving, it is referred to as choke feeding. During choke feeding the crushing action takes place between the jaw plates and particles as well as by inter-particle compression. Choke feeding necessarily produces more fines and requires careful feed control. For mineral liberation, choked feeding is desirable.

When installed above ground, the object of the crushing circuit is to crush the ore to achieve the required size for down stream use. In some industries, for example, gold ore ,iron ore or coal, where a specific product size is required (iron ore -30+6 mm), careful choice of jaw settings and screen sizes are required to produce the minimum amount of fines (i.e. - 6 mm) and maximum the amount of lump ore within the specified size range. For hard mineral bearing rocks like gold or nickel ores where liberation of minerals from the host rock is the main objective, further stages of size reduction are required. closed circuit flow sheets showing in fig3.